Area Forecast Discussion
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FXUS61 KLWX 230117
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
917 PM EDT Wed Jun 22 2016
Low pressure will approach from the Ohio Valley tonight and then
move across the region Thursday. High pressure will build into the
area Thursday night and remain through the weekend. A cold front
will approach the mid-Atlantic early next week.
.NEAR TERM /THROUGH THURSDAY/...
At present...a frontal boundary remains draped across central
Virginia. Much drier and more stable air mass in place across most
of the region at present. (The 00z LWX RAOB has 0 CAPE, while the
sounding from RNK has SB/MU CAPE almost 1750 j/kg.) It still looks
like the front will try to move back northward in advance of low
pressure moving eastward from the midwest. This low will pass near
the area during the day Thursday.
There is still more than the desired uncertainty regarding severe
weather and flooding, mainly due to the presence of the
aforementioned front. Some guidance (12Z GFS in particular) shoves
the front back north late tonight and early Thursday, resulting
in an extremely unstable atmosphere on Thursday afternoon (4000+
j/kg cape) from which strong to severe storms would be likely.
However, this would not seem particularly likely if the idea of
an MCS moving across the region is correct...in which case the
rain cooled air would likely be stuck in a wedge east of the
mountains and the front would be unable to push back north.
The other question revolves around said MCS, which may or may not
bring a round of severe weather later tonight. With the front
sagging southward still and a stable air mass in place, any MCS
moving in from the west could just ride right over the stable air
mass and start weakening, which would reduce the severe threat.
However, should the front manage to slide back northward late
tonight ahead of the MCS, it could tap the more unstable air to
the south directly and bring another round of severe thunderstorms
and heavy rain.
Right now (01Z), it appears as though the MCS is organizing in
northern Illinois. It has a long way to travel (nearly 600 mi)
before it affects the east coast. Most of the recent HRRR/RAP/LWX
WRF-ARW4 runs as well as the 18z GFS/NAM all suggest
precipitation arriving in the Applachians somewhere around 09z,
and to I-95 near 12Z (give-or-take). That is a logical
progression based on radar trends. Have therefore pulled all
mention of rain from the forecast for the evening, and
reintroduced it toward morning. It remains to be seen how much
instability will be had at that hour, or whether the in-storm
environment will render that a moot point. Can`t rule out heavy
rain or gusty winds...but am delaying them a bit.
The Flash Flood Watch remains in place for the Potomac Highlands.
These areas have low flood guidance values and have the highest
certainty of impact prior to potential weakening. Beyond that,
it`s not completely clear whether the complex will track east or
dive south. Therefore, am not making any changes at this time.
Jury is still out with respect to Thursday; the early morning
activity will play a role. However, it seems as though the focus
/may/ be on our Virginia counties as opposed to Maryland. But,
that depends on what happens to the front behind the likely-
decaying MCS as well as how much recovery transpires...so there is
plenty of room for that projection to change.
Made no changes to the temperature forecast at this time. Lows
tonight will be mostly in the 60s. Highs on Thursday are uncertain
but are forecast mainly in the upper 70s and 80s. Depending on
where the front ends up...this forecast could be off by some
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
Front drops south of the area Thursday night and remains south
through Saturday. High to the north will provide northeast to east
flow across the region in its wake. Upslope flow combined with
trough aloft may allow isolated to scattered showers and perhaps
even a stray thunderstorm mainly near the mountains...and
especially further south. Otherwise it will be rather pleasant.
The threat of showers will diminish Saturday.
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
500 MB height rises through Sunday night before zonal flow sets
up through the Monday night. H5 trough and associated surface cold
front approaches late Monday with frontal passage in the
Tuesday/Tuesday night time frame. Best chances of
shower/thunderstorms will be from Monday afternoon through Tuesday
Maximum/minimum temperatures and POPs are close to normals for
early summer. No outstanding heat or humidity issues in the
forecast at this moment.
.AVIATION /01Z THURSDAY THROUGH MONDAY/...
Still big uncertainty early Thursday regarding severe weather
threat...but at least through midnight we should generally stay
VFR at all terminals. The only substantive change was to delay the
primary threat window closer to 12z. Early morning thunderstorms
can bring reductions in vis and cigs to IFR...along with damaging
There may or may not be a second round Thursday afternoon. It
will depend on the track/timing of the morning convective complex,
the positioning of the stalled boundary behind the complex, and
how much recovery we can experience during the middle of the day.
Improving conditions generally Thursday night and VFR Friday -
MVFR conditions possible early next week for all terminals with cold
Winds overnight will be light. Storm complex moving in from the
west will bring rain, thunderstorms and another round of SCA
winds on Thursday. Conditions improve Friday-Saturday.
In the extended time frame, best chances of small craft conditions
will be on Monday pre-frontal, and then again on Tuesday post-
MD...Flash Flood Watch from 2 AM EDT Thursday through Thursday
afternoon for MDZ003-501-502.
VA...Flash Flood Watch from 2 AM EDT Thursday through Thursday
afternoon for VAZ027-028-030-031-503-504.
WV...Flash Flood Watch from 2 AM EDT Thursday through Thursday
afternoon for WVZ050>053-055-501>506.
MARINE...Small Craft Advisory from 6 AM to 6 PM EDT Thursday for